NFC technologies are described in the prior published specifications of the NFC Forum, which may be found at “http://www.nfc-forum.org/specs/” and are incorporated into this specification by reference. In general, NFC communication takes place at a frequency of 13.56 MHz over a range of less than 20 cm, more usually 10 cm or less. It enables an exchange of data between at least two devices, one of which, an initiator, is generally acting as an active read device (reader), to acquire data from at least one other target (passive) device, acting as a listen device.
The communication is achieved by the initiator device emitting an RF magnetic field from an antenna coil which activates any NFC enabled target device in range. The initiator then sends a digitally encoded poll request by amplitude modulating the RF field. This is followed by the initiator generating an un-modulated carrier field for a specified period. If the target device is compatible, it responds by load modulating the un-modulated RF field to transmit a digitally encoded signal to the initiator. An exchange of digitally encoded signals follows culminating in the desired data communication. Load modulation is similar to a shallow AM modulation with a modulation index usually less than 10%.
It is becoming commonplace to integrate NFC communication capability in smart devices such as: mobile phones, tablet computers and like mobile devices. Such devices are thus NFC enabled and may behave optionally as either initiator or target according to the instant user desired functionality.
In order to NFC enable a cell phone, and maintain economies of fabrication, compact design and efficient energy usage, the various circuits and assemblies which NFC enable a cell phone exist in close proximity to circuits and assemblies which facilitate other connectivities such as: 2G, 3G and 4G communication, Bluetooth®, WiFi (IEEE 802.11x) FM radio, global positioning systems, and data processing, by way of non-exhaustive examples. It is common that the circuits for these functionalities cohabit on the same substrate (chip) and share the same signals, power lines, ground lines, and clock trees. This fact allows parasitic (unwanted) electro-magnetic coupling between the cohabiting circuits. The intrinsic nonlinearity of active initiator NFC devices causes the emission of out-of-band disturbances (mainly higher order signal frequency harmonics). These disturbances induce a severe distortion, especially in the other wanted analog signals (called “aliasing”), and the distorting higher order signal frequency harmonics are called “aliases”, (aliases are unwanted signals, particularly higher order harmonics). The disturbances also cause DC shift errors on sensitive nodes in the victim circuit, thereby driving the latter out of its correct operation mode. The aliases of the NFC carrier (13.56 MHz) can fall in the carrier bands of the smart device's other connectivities. These connectivities are then referred to as victims of aggressor NFC.
For active initiator NFC, so far, EMC (Electro Magnetic Compatibility) filters have been used to reduce the aforementioned cohabitation problems created by the initiator NFC device. A prior art EMC filter is illustrated in FIG. 1. The EMC filter 1 is a purely analog anti-aliasing filter inserted between a matching circuit 2 of an antenna 3 and network transmission terminals 4 of the active NFC device. The filter reduces the spectral density of the aliases.
In the device acting as the target (i.e. in tag-emulation mode), EMC problems arise resulting from the harmonics (sidebands) of the aforementioned load modulation. The load-modulation frequency is n*(13.56/128) MHz is such that n=[1, 2, 4, 8], the sidebands are located at N*n*(13.56/128) MHz where N=[1, 2, 3, . . . ].
A conventional EMC filter designed to filter out the 13.56 MHz harmonics is not at all efficient in filtering out the out-of-band harmonics (sidebands) of the much lower load-modulation frequency (n*[13.56/128] MHz). On the other hand, it is not possible to add a second EMC filter (for n*(13.56/128) MHz harmonics filtering) because it will filter out the carrier signal at 13.56 MHz. Filtering out the carrier signal results in filtering the transmitted data in both initiator (active) and target (passive/tag) emulation modes, which is of course undesirable.
A conventional circuit for load modulation of an NFC enabled device in target emulation mode is shown in FIG. 2. Modulation is achieved when a digital modulator 9 drives a switch 7, which intermittently connects a resistance 6 across the terminals TX1, TX2 of the EMC filter 2. The digital modulator is driven by “other blocks” which can be understood as an NFC controller such as a dedicated or general purpose processor running machine code to implement NFC. The resistance 6 has a single time independent value under constant operating conditions and will effect a change in the peak voltage seen at the aerial circuit terminals TX1 and TX2.
FIG. 3 is a graph of the voltages measured at the terminals 4 of the target (tag) EMC. The voltage for these purposes is the peak voltage of the carrier wave which is amplitude modulated to carry a digitized signal. In the prior art example of FIG. 3 a high voltage represents digital zero, while a low voltage represents digital one.
In general the modulation resembles a rectangular wave function (rect(t)) with the transition between low and high voltage occurring instantaneously. However, where the resistor switching occurs to change the digital value between “0” (˜3.0-3.5V) and “1” (˜1.5V) fast jumps occur which are equivalent to (indicative of) higher harmonics of the frequency of load modulation. That is to say, the frequency of the clock that generates load modulation data=n*(13.56/128) MHz). A fast time-domain jump (i.e. a rectangular time-domain signal: rect(t)) has a cardinal sine (sinc(πf)) spectrum in the frequency domain. This means that the spectrum is not limited in the frequency domain because the cardinal sine spectrum is a spectrum where most of the energy of the signal is concentrated in the main lobe of the cardinal sine (this is the in-band, wanted part of the signal), and a tiny part of the energy of the signal is found in the side-band lobes (of the cardinal sine) that act as out-of-band unwanted harmonics which can aggressively interfere with victim connectivities.